In a multi-user (MU) multiple-input multiple-output (MIMO) system, a base station with multiple transmit antennas simultaneously sends different information streams to different mobile terminals, each with one or more receive antennas. The base station aims to orthogonalize the different information streams, and thus reduce interference among them, by precoding the downlink MU-MIMO transmission. In order to determine a precoding that accounts for the downlink MU-MIMO channel conditions, the base station must receive timely channel state information (CSI) feedback from the mobile terminals.
Because the uplink bandwidth for feedback is limited, the mobile terminals typically only feed back partial CSI. In one approach, each mobile terminal feeds back quantized channel coefficients. In another approach, each mobile terminal feeds back information that merely describes channel statistics. Most implementations of the latter approach further limit feedback to either the channel mean or the channel covariance. The few implementations that feed back comprehensive statistical information, feeding back both the channel mean and covariance, are prohibitively complex in practice.
This remains especially true in a coordinated multi-point (CoMP) (i.e., multi-cell) context, which presents additional complexities. In a centralized CoMP architecture, geographically distributed base stations connect to a CoMP controller. The CoMP controller jointly precodes MU-MIMO transmissions simultaneously sent from the base stations to mobile terminals. This joint precoding certainly increases achievable rates, but the accompanying increase in complexity has nonetheless prohibited known joint precoding approaches from reaching the maximum rates theoretically possible.